There are a variety of rack mount enclosures currently available which draw external ambient air from the front of the enclosure, pass the air through the enclosure and exhaust the air through the rear of the enclosure. As the moving air passes by operating circuits within the enclosure, the air carries away heat from the operating circuits thus maintaining the operating circuits within a normal operating temperature range for proper operation and reliability.
One conventional rack mount enclosure is 1 U (approximately 1.75 inches) in height and includes a fan assembly configured as a field replaceable unit (FRU). The fan assembly includes a fan frame and a row of four fans fastened to the fan frame. The fan frame includes a vertical face plate which faces outwardly from the front of the enclosure and two thumbscrews secured to the vertical face plate. The vertical face plate and the thumbscrews are offset from the profiles of the four fans in order to avoid obstructing the airflow generated by the fans. The thumbscrews thread into thumbscrew holes defined by the enclosure thus holding the frame to the enclosure to prevent the fan assembly from inadvertently escaping (e.g., due to vibration).
Suppose that an installed fan assembly suffers a fan failure while out in the field. To replace the installed fan assembly with a new fan assembly, a field technician unscrews the thumbscrews from the enclosure and pulls the installed fan assembly out of the enclosure. The field technician then slides the new fan assembly into the enclosure and screws the thumbscrews of the new fan assembly into the enclosure. If such fan assembly replacement occurs within a short amount of time (e.g., 30 seconds), the temperature of the operating circuits within the enclosure will remain within the normal operating temperature range thus alleviating the need to turn off the operating circuits to prevent overheating during fan assembly replacement.